Two-stage reciprocating pumps are already known in which the two pistons are joined with each other by means of a piston rod and are driven by means of a linear drive (see the brochure "LABOVAC-Linear-Membranpumpen und Kolbenpumpen" [LABOVAC Linear Diaphragm Pumps and Reciprocating Pumps] from the firm of SASKIA, Hochvakuum--und Labortechnik GmbH [SASKIA High-Vacuum and Laboratory Engineering Co.], O-6300 Ilmenau, Germany). It is also mentioned there that, with special models, it is possible to achieve hermetic sealing of the pistons through the installation of a separating membrane. However, reciprocating pumps of this type, with or without a separating membrane, still have several disadvantages:
In the case of pistons causing expulsion, for example into open air, condensation can occur if there is corresponding moisture in the pumping medium. This leads to increased wear and to leakage at the piston seals. That means a decline in performance for the entire pumping unit.
Also already known is a reciprocating pump in which the piston-cylinder space is closed off down to the crank area by means of a sealing membrane. This prevents atmospheric air, for example, from being able to get past the piston rings or a lip seal of the piston and thereby somewhat degrading the vacuum generated in the reciprocating pump. This also prevents the disadvantage of the actual pumping medium itself becoming contaminated by possibly contaminated air coming from the crank area. It must also be noted that over the long run, a seal cannot be attained where the crankshaft comes through, and that because of the mechanical movement, some form of lubrication is necessary in the crank area. If the piston-cylinder space is not sealed off from the crank area, this also can contribute to unwanted impurities in the actual pumping medium.
From the brochure "LABOVAC D65-D1600" of the firm of SASKIA Hochvakuum--und Labortechnik GmbH, there is already the suggestion to use a linear-acting, two-stage reciprocating pump provided with two sliding pistons, as described above, as a fore-pump for a turbomolecular pump. However, this also brings several disadvantages with it. First of all, the known two-stage reciprocating pump with linear drive has the above-mentioned disadvantage of condensate formation. Secondly, it has no balancing of mass with respect to the movements of the pistons, or an expensive, additional mass balance must be provided.
When such a known two-stage, linear reciprocating pump works in conjunction with a turbomolecular pump, the usual vibrations lead to unwanted movements of the turbomolecular pump, which is usually joined with the two-stage reciprocating pump in a single frame, or is even made as a common pump block. The turbomolecular pump is, however, very sensitive to vibration. As is known, turbomolecular pumps of the known design types exhibit rotational speeds of, for example, 30,000 rpm or even rotational speeds that are substantially higher. The rotors of such turbomolecular pumps are therefore usually mounted in magnetic bearings, and are correspondingly sensitive to shocks.